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The basic dipole shape of Earth's magnetic field (bottom) and the teardrop shape of the magnetosphere created by the solar wind (top).
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Earth's Magnetic Field

Earth has a magnetic field. If you pretended that Earth had
a gigantic bar
magnet inside of it (it doesn't really, of course), you would
have a pretty good idea about the approximate shape of Earth's magnetic field.
Earth's magnetic field is slightly tilted with respect to the planet's spin
axis; there is currently a difference of about 11° between the two. Because
of this difference, the Geographic North Pole and the North Magnetic Pole
are not actually in the same place; likewise for the South Poles. This means
that
compasses
do not always point directly towards True North.

Although scientists do not understand all of the details, they know that motions
of molten metals in the Earth's core generate our planet's magnetic field.
Movement of molten iron and nickel generates electrical and magnetic fields
that produce Earth's magnetism. The flows of these molten metals in Earth's
outer core are not perfectly steady over time, so Earth's magnetic field changes
over time as well. The North and South Magnetic Poles wander over time; the
North Magnetic Pole moved some 1,100 km (684 miles) during the 20th century.
The strength of Earth's magnetic field varies as well; it has been decreasing
slightly ever since around 1850. Over the course of Earth's history the magnetic
field has actually reversed itself many times, with North becoming South and
vice versa!

Earth's magnetic field extends thousands of kilometers (miles) outward into
space. The field forms a gigantic magnetic "bubble" in space around Earth.
This magnetic bubble is called the magnetosphere. Earth's magnetosphere shields
our planet from most particle
radiation that flows our way from the Sun and
other radiation sources in space. The magnetosphere is not actually a sphere;
it is shaped more like a teardrop, with a long "tail" extending away from the
Sun.

Although Earth's magnetic field is roughly a dipole (like the field of a bar
magnet) to a first approximation, it has a much more complex shape than a simple
dipole field. The uneven flows and distributions of the molten metals that
generate Earth's field cause the field to be quite "lumpy". The pressure of
the solar wind, the stream of charged particles flowing outward from the Sun,
also distorts the shape of the magnetic field surrounding Earth.

The names given to the Magnetic North and South Poles are potentially quite
confusing. Recall that opposite poles of magnets attract, while like poles
repel each other. If you take two bar magnets, and place their North Poles
near each other, they will push themselves apart; likewise for two South Poles.
If you place a North Pole near a South Pole, they will pull themselves together.
The needle of a compass is a small bar magnet, with a North and a South Pole.
The North Pole of the compass needle points North (roughly). But the North
Pole of a magnet is attracted to a South Pole of another magnet. So Earth's
North Magnetic Pole is actually a South Pole of a magnet!

Several other planets, and even a few moons, in our Solar System also have
magnetic fields. Our Moon has a very weak magnetic field, as does the planet
Mars. Mercury's field is a bit stronger. The giant planets Jupiter and Saturn have extremely powerful fields. Uranus and Neptune also have fairly strong
fields. We don't know about Pluto yet, but it is unlikely to have a strong
field if it has one at all. Venus does not have a magnetic field, probably
because it rotates so slowly. Jupiter's moon Ganymede also has a magnetic field,
and we have tentative hints that some other moons may have weak fields as well.

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